CANVAS and otoneurological evaluation: seeing the whole picture

Background: The syndrome defined by cerebellar ataxia, neuropathy and vestibular areflexia (CANVAS) was previously described as a cause of late-onset ataxia. With the discovery of biallelic expansion in the RFC1 gene as its underlying genetic cause, CANVAS and the broader RFC1-disease became more clinically heterogeneous and one of adult’s most common genetic cause of ataxia. Methods: Retrospective cohort comprising patients with a genetic diagnosis of CANVAS. Data related to neurological examination, video Head Impulse Test (VHIT), caloric tests, posturography, electromyography/nerve conduction studies and brain magnetic resonance imaging (MRI) were considered. Results: We included 15 patients, with a current mean age of 65.8±12.8 years. Ten (66.6%) patients presented with the complete clinical triad. At neurological examination, 13 patients showed signs of peripheral neuropathy. Cerebellar dysfunction was observed in 12 patients, whereas postural instability was seen in 11 patients. Electromyography/nervous conduction studies revealed peripheral neuropathy in all of the cases. Bilateral vestibular dysfunction was confirmed in approximately one-half of the patients. Mean balance values from the posturography were decreased in the majority (n=14). In the imaging assessment (n=11), 6 patients displayed significant vermian atrophy, predominantly in the anterior/dorsal regions, while the other 5 patients showed moderate atrophy. Conclusions: With the discovery of the genetic cause of CANVAS, it was possible to identify many patients in the absence of the classic triad. Detailed characterization/phenotyping through clinic and multimodal integration if the complementary diagnostic tests, allows a better understanding of the entire spectrum of RCF1- related disorder. not-yet-known not-yet-known not-yet-known unknown CANVAS and otoneurological evaluation: seeing the whole picture not-yet-known not-yet-known not-yet-known unknown Abstract Background: The syndrome defined by cerebellar ataxia, neuropathy and vestibular areflexia (CANVAS) was previously described as a cause of late-onset ataxia. With the discovery of biallelic expansion in the RFC1 gene as its underlying genetic cause, CANVAS and the broader RFC1-disease became more clinically heterogeneous and one of adult’s most common genetic cause of ataxia. Methods: Retrospective cohort comprising patients with a genetic diagnosis of CANVAS. Data related to neurological examination, video Head Impulse Test (VHIT), caloric tests, posturography, electromyography/nerve conduction studies and brain magnetic resonance imaging (MRI) were considered. Results: We included 15 patients, with a current mean age of 65.8±12.8 years. Ten (66.6%) patients presented with the complete clinical triad. At neurological examination, 13 patients showed signs of peripheral neuropathy. Cerebellar dysfunction was observed in 12 patients, whereas postural instability was seen in 11 patients. Electromyography/nervous conduction studies revealed peripheral neuropathy in all of the cases. Bilateral vestibular dysfunction was confirmed in approximately one-half of the patients. Mean balance values from the posturography were decreased in the majority (n=14). In the imaging assessment (n=11), 6 patients displayed significant vermian atrophy, predominantly in the anterior/dorsal regions, while the other 5 patients showed moderate atrophy. Conclusions: With the discovery of the genetic cause of CANVAS, it was possible to identify many patients in the absence of the classic triad. Detailed characterization/phenotyping through clinic and multimodal integration if the complementary diagnostic tests, allows a better understanding of the entire spectrum of RCF1- related disorder. Acknowledgment Not applicable.

RFC1, cerebellar ataxia, neuropathy, vestibular areflexia syndrome, sensory neuropathy. not-yet-known not-yet-known not-yet-known unknown Keypoints: CANVAS is one of adult’s most common genetic cause of ataxia. Discovery of RCF-1 expansion enabled CANVAS diagnosis without the classic triad. Posturography has the potential to detect subclinical postural instability. MRI may reveal significant vermian atrophy, predominantly in the anterior/dorsal regions. Multimodal phenotyping aids in understanding the RCF1-related disorder spectrum. Introduction Cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) has been characterized as a form of slowly progressive, late-onset ataxia 1-3. This condition arises from any combination of impairments in the vestibular system, peripheral nerves (dorsal root ganglia), and cerebellum, inevitably leading to ataxia 2. Although cases of combined ataxia and vestibulopathy were first reported in the 1990s, CANVAS was formally identified as a distinct clinical entity in 20114,5. In 2019, Cortese et al. discovered that biallelic expansions in the replication factor C subunit 1 (RFC1) gene are the genetic cause of CANVAS, a finding that revolutionized the diagnostic approach to this autosomal recessive syndrome 6. This discovery allows for diagnosis even in the absence of the complete classic triad, thereby redefining the condition as an RFC1-related disorder 6,7. The phenotypic spectrum associated with biallelic intronic repeat expansions in RFC1 includes a range of symptoms such as typical cerebellar ataxia, sensory neuropathy, and vestibular areflexia syndrome (CANVAS); subclinical impairments in cerebellar, sensory, and vestibular functions; autonomic dysfunction; chronic cough; and more limited phenotypes primarily affecting one of the systems involved in balance control. Additional neurological features reported within the RFC1 spectrum include motor neuropathy, hyperkinetic movement disorders, parkinsonism, REM and non-REM sleep disorders, and cranial neuropathy 8,9. The shift from a predominantly clinical-based diagnosis to genetic testing has facilitated more accurate diagnoses, positioning RFC1-related disorders as a major genetic cause of adult-onset cerebellar ataxia and a significant contributor to idiopathic sensory neuropathy 2,7,10. Despite these genetic advancements, neurological examination remains crucial for identifying symptomatic individuals potentially affected by this condition, revealing vestibular, cerebellar, and sensory impairments. Our objective is to enhance the understanding of the phenotypic spectrum directly caused by RFC1 expansion through an interdisciplinary approach, including neurological, otoneurological, and neuroimaging evaluations. Methods Patient data Retrospective analysis of a cohort of patients followed in a tertiary center. Included all patients, followed in Neurology and/or Otorhinolaryngology consultation, who had the biallelic pentanucleotide repeat expansion in the gene encoding RFC1 . Patients records were retrospectively evaluated, including family history, clinical and/or evidence of peripheral neuropathy, cerebellar and/or vestibular dysfunction. All patients were clinically classified according three main axis: peripheral neuropathy, vestibular impairment and cerebellar disfunction. Sensory neuropathy/neuronopathy was inferred by the presence of sensory symptoms (loss of feeling, paresthesia, dysesthesia, neuropathic pain) and/or abnormal sensory exam, including sensory ataxia. Abnormal head impulse test (HIT) and/or oscillopsia were used to access the presence of vestibulopathy. Cerebellar dysfunction was deduced from neurological examination findings, such as cerebellar dysarthria, dysmetria and/or abnormal eye movements (nystagmus, dysmetric saccades, impaired ocular pursuit). Additionally, vestibular impairment was further evaluated through video Head Impulse Test (VHIT), caloric test and posturography. Data from electromyography/nerve conduction studies and brain MRI were also collected. The study was approved by local institutional ethical committees. RFC1 pentanucleotide repeat expansion testing Biallelic AAGGG repeat expansion in RFC-1 was searched by polymerase chain reaction. Electrodiagnostic testing Nerve conduction studies were conducted using standard techniques, encompassing motor nerves (median, ulnar, tibial, peroneal) and antidromic sensory nerves (median, ulnar, radial, sural, and/or superficial peroneal). Concentric needle electromyography was performed based on clinical discretion. All electrophysiological assessments were carried out using a Keypoint® G4 system. A qualitative analysis was provided based on the comprehensive findings. Vestibular testing Video Head Impulse Testing (vHIT) was performed using the Otometrics ICS Impulse® system, documenting gains across all six semicircular canals as well as overt and covert saccades. Videonystagmography (VNG) was recorded using the VNG Ulmer, SYNAPSIS® system. Bithermal caloric tests were conducted by stimulating the external auditory canals with cold (30ºC) and warm (44ºC) water. Bilateral vestibulopathy was defined according to the diagnostic criteria consensus of the Bárány Society for bilateral vestibulopathy, published in 2017 11. Specifically, bilateral vestibulopathy was diagnosed if the horizontal canal angular vestibulo-ocular reflex gain was less than 0.6 on vHIT, or if the sum of the bithermal maximum peak slow phase velocities on each side was less than 6º/sec in the caloric test. Posturography Computerized Dynamic Posturography was conducted using the NeuroCom EquiTest System®. The results of the Sensory Organization Test and the Limits of Stability were documented. MRI Brain MRI examinations were conducted using standard clinical protocols on 1.5T and 3T scanners. Coronal T2-weighted, 3D fluid-attenuated inversion recovery (FLAIR), and 3D T1 sequences were employed to assess cerebellar atrophy, with a qualitative description provided for each evaluated region. All images underwent meticulous review by two experienced neuroradiologists. Statistics Statistical analysis was performed using SPSS version 27. Data were expressed as means ± standard deviations (SD).

Among the 15 patients harboring biallelic AAGGG repeat expansions in the RFC1 gene, 8 (53.3%) were male. Demographic and clinical characteristics are summarized in Table 1. The cohort had a mean age of 65.8 ± 12.8 years, with a mean age of clinical onset at 53.9 ± 12.0 years. Imbalance was reported as the initial neurological symptom in 11 (73.3%) subjects. Family history suggestive of inherited neuropathy was noted in 8 (53.3%) patients. Chronic idiopathic cough was present in 8 patients (53.3%). At the last assessment, 10 patients (66.6%) exhibited the classical triad of CANVAS symptoms. Complaints of liquid dysphagia were reported by 6 (40.0%) patients. Additionally, other related symptoms/complaints were evaluated, including constipation (n=2), sexual dysfunction (n=1), and urinary incontinence (n=1). All patients underwent evaluation by a senior neurologist specializing in Neuromuscular disorders, and findings from previous neurological examinations are summarized in Table 2. At the latest assessment, 13 patients (86.7%) presented with distal length-dependent sensory deficits, including 2 patients with concurrent distal motor deficits. The majority (n=11, 73.3%) had bilaterally abnormal Head Impulse Tests (HIT), and 5 (33.3%) patients exhibited cerebellar ocular motor signs, such as downbeat nystagmus (n=3), simultaneous jerky smooth pursuit and saccadic dysmetria (n=1), and alternant strabismus (n=1). Skew deviation on the cover test was noted in one patient. Knee reflexes were elicitable in 6 patients, reduced or absent in 7 patients, and brisk in 2 patients. Ankle reflexes were more frequently reduced or absent (n=10), but preserved reflexes were observed in 5 patients. Cerebellar ataxia was evident in 13 patients (86.7%), with only 5 (33.3%) requiring unilateral assistance despite exhibiting ataxic gait. Electrodiagnostic assessments (Table 3) identified peripheral neuropathy in all subjects. Sensory axonal polyneuropathy was found in 7 (46.7%) patients, sensory neuronopathy in 4 (26.7%), and motor and sensory axonal polyneuropathy in 3 (20.0%). Vestibular function tests were performed in all patients (Table 4). Abnormalities in vHIT were noted in 11 patients (73.3%), with 8 (53.3%) meeting criteria for bilateral vestibular hypofunction and 3 (20.0%) displaying unilateral involvement. Among the 6 patients diagnosed with bilateral vestibulopathy on caloric testing, all also exhibited bilateral hypofunction on vHIT. Mean gains on all six canals during vHIT were reduced (Figure 1), and overt and covert saccades were frequently observed. Regarding posturography (Figure 2), global values of the Sensory Organization Test (SOT) were reduced in the majority (n=14; 93.3%), with average results notably decreased in conditions 2 to 6. Multisensory deficits and visual dependence patterns were commonly observed. End-point excursion (EPE) and maximum excursion (MXE) regions of stability were globally reduced. Brain MRI data (Table 5) were available for 11 (73.3%) patients. Marked vermian atrophy with anterior/dorsal predominance was observed in 6 (40.0%) patients, while moderate atrophy was noted in the remaining 5. The degree of atrophy in Crus I varied widely, with severe patterns in 3 patients, moderate in another 3, mild in 3, and non-valuable in 2 (13.3%). These patterns of cerebellar atrophy distribution are illustrated in Figure 3.

Advancements in understanding CANVAS have markedly accelerated in recent years, particularly following the discovery of the RFC1 gene in 2019 2. Although the precise pathogenic mechanisms underlying biallelic RFC1 expansions remain elusive, the ability to achieve a molecular genetic diagnosis has positioned CANVAS as one of the most prevalent late-onset ataxias 2,10 . This molecular insight has significantly improved diagnostic sensitivity and specificity, enabling earlier identification of patients who may present with partial manifestations of the triad 2,3 Notably, peripheral neuropathy was universally identified in electrodiagnostic studies and could be detected on physical examination in the majority (86.7%) of patients. Clinical cerebellar dysfunction was present in a substantial majority (80%) of patients, with corresponding imaging evidence of atrophy observed in all examined brain MRIs. Similarly, a significant proportion (73.3%) of patients exhibited vestibular impairment, confirmed either clinically or through vHIT/caloric testing, although criteria for bilateral vestibulopathy were met in only 8 (53.3%) cases. The observed concordance rate between HIT and vHIT was 73.3%. Dynamic posturography offers a more objective quantification of the balance and how a patient use its sensory systems ((proprioceptive, visual, and vestibular) to maintain stability. Instability is an usual finding in patients with CANVAS, as it was observed in the series. In fact, dynamic posturography revealed reduced global values in the Sensory Organization Test (SOT) in the majority (93.3%) of patients. Noteworthy, multisensory deficits and patterns of visual dependence were frequently observed within the cohort. As suggested by other, we advocate that posturographic studies should be a part of the evaluation of a patient with instability of any origin 12 . From a clinical standpoint, the classical triad of CANVAS symptoms was evident in only 10 patients (66.6%). However, this number increased to 11 (73.3%) upon comprehensive neurological examination and to 12 (80.0%) when including ancillary exams. As previously reported, RFC1-related disorder manifests as a slowly progressive condition, with the majority of patients exhibiting ataxic gait, although only 5 required unilateral assistance. Those requiring assistance tended to have a longer disease duration (14.0 ± 6.51 months vs. 9.8 ± 8.69 months, p=0.360). Dysphagia was noted in 6 patients (40.0%) and was positively associated with disease duration (p=0.003), serving as an indicator of disease severity primarily related to cerebellar involvement. None of the patients required percutaneous endoscopic gastrostomy due to manageable mild to moderate dysphagia using behavioral measures and thickeners. Our findings are consistent with those of a larger clinical cohort of CANVAS patients from the UK, albeit without inclusion of posturography 7 . Recent studies have predominantly focused on clinical neurological examination findings. Integration of otoneurological assessments, particularly vHIT, caloric tests, and posturography, provides a comprehensive evaluation of vestibular dysfunction. We advocate for a multimodal phenotyping approach essential for elucidating the full spectrum of phenotypes associated with RFC1 expansion. Another strength of our study lies in the cohort’s homogeneity, comprising exclusively RFC1-positive patients, in contrast to clinical-defined CANVAS cohorts 13-16 . A notable limitation is the small size of our cohort, reflecting the rarity of the disease and its recent recognition, potentially leading to underdiagnosis. Nonetheless, due to the uncommon nature and recent characterization of RFC1 disease, our findings contribute novel insights. Additionally, the length of pentanucleotide repeat expansions was not quantified, precluding assessment of potential associations with disease severity and age of onset, as suggested in recent literature 17 . Finally, the retrospective nature of our study represents a significant limitation, dependent on the quality of clinical records.

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May 3 2024;147(5):1887-1898. doi:10.1093/brain/awad436 not-yet-known not-yet-known not-yet-known unknown LA left anterior SSC, LL left lateral SCC, LP left posterior SCC, RA right anterior RL right lateral SCC, RP right posterior SCC Figure 1: VHIT gains on the six semicircular canals Figure 2. Sensory organization test average results (conditions 1 to 6). not-yet-known not-yet-known not-yet-known unknown Figure 3. Brain MRI: 3D sagittal T1-weighted image illustrating a characteristic pattern of cerebellar atrophy on CANVAS. Atrophy of the anterior and dorsal cerebellar vermis, between the primary and prepyramidal fissures, corresponding to VI, VIIA and VIIB lobules (A); and lateral hemispheric, predominantly affecting the Crus I lobule (B), which is translated by a slight enlargement of the posterosuperior (gray arrow) and horizontal (red arrow) fissures. Information & Authors Information Version history Copyright This work is licensed under a Non Exclusive No Reuse License. Authors Metrics & Citations Metrics Article Usage 167views 81downloads Citations Download citation André Aires Fernandes, Pedro Alexandre, Sofia Vedor, et al. CANVAS and otoneurological evaluation: seeing the whole picture. Authorea. 10 March 2025. DOI: https://doi.org/10.22541/au.174159952.27400179/v1 DOI: https://doi.org/10.22541/au.174159952.27400179/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.